The invention relates to an arrangement for controlling the power transmission to at least two axles of a motor vehicle and more specifically for improved control of a friction element for limiting rotational speed compensation between the axles.
In order to avoid distortions in the drive line between both axles, motor vehicles with a permanent all-wheel drive require a differential device, such as a center differential. This allows rotational speed differences between the wheels of both axles to occur, for example, during cornering or in the case of tires with different tread radii.
When only one axle loses static friction between the wheels and the road surface, however, it is known that as a result of the differential effect of the center differential, the driving power of the whole drive is lost. Thus the use of an all-wheel drive of this type does not seem to be very useful without a device for limiting the differential effect such as for example the use of a differential lock or brake.
It is known to use so-called visco-clutches or center differentials that are torque responsive (e.g. limited slip differentials with worm gears) for limiting the differential effect. These automatically operating elements have disadvantages resulting from their basic principle, since they may become operative even during tight cornering or when tread radii of the wheel are very different. Thus they prevent a rotational speed difference between the axles, which is required with respect to the dynamics of vehicle movement. However, this limits the maneuverablity of the vehicle particularly on a smooth road surface, and any narrow cornering can bring on the so-called "narrow-corner braking".
On the basis of DE-OS No. 35 05 455, an arrangement has become known for the automatic connecting and disconnecting of driving elements of a motor vehicle. In this case, the motor vehicle is normally driven by means of only one axle. When certain threshold values are reached with respect to dynamics of vehicle movement such as a certain speed slip between the wheels of two axles, a second driving axle, a differential gear lock, a rear axle differential lock and a front axle differential lock are then connected in steps by a shiftable clutch and a differential gear.
It is known from this text that the threshold values for the connecting and disconnecting of the driving elements must be very low, so that an effectiveness is achieved that is sufficient with respect to the dynamics of vehicle movement and thus an improvement of the vehicle handling in comparison to conventional driving systems is obtained. However, this requires a cornering recognition and correction which, in this case, takes place by a steering angle sensor.
For the recognition of different tread radii of the wheels, an electronic adaptation system is also provided which corrects the measured wheel rotational speed values correspondingly.
However, the connecting and disconnecting of a vehicle axle, because of the advantages with respect to the dynamics of vehicle movement (directional control) offered by a permanent all-wheel drive, does not seem to be very useful. Likewise, an abrupt connecting and disconnecting of driving components can be used only in vehicles having a large inertial mass, because this connecting or disconnecting, always results in a "jerk" and otherwise have a negative effect on vehicle handling comparable to a load change reaction.
In addition, the correction of the wheel rotational speed values by the electronic adaptation system results in the difficulty that although the connecting of the driving components takes place correctly, the disconnecting of the locks under certain operational circumstances can no longer be carried out solely as a function of the slip. The reason for this is that when the lock is completely connected, it prevents rotational speed differences of the wheels, in which case it is unimportant whether the rotational speed differences are caused by different tread radii of the wheels or by road grip. The corrected wheel rotational speed values also make the decision to disconnect more difficult.
It is therefore the object of the invention to provide an arrangement for controlling the power transmission to at least two axles of a motor vehicle for limiting rotational speed compensation between the axles that requires a minimal input of sensed operating and/or driving parameters and at the same time entails low expenditures with respect to measuring and control techniques. A further object of the present invention is to provide a power transmission control that has an optimal overall performance with respect to the dynamics of motor vehicle movement in almost all driving situations.
The advantages of the invention are mainly that an arrangement for controlling the power transmission to at least two axles of a motor vehicle is provided through a friction element that limits a rotational speed compensation between axles with a low input of sensed operational and/or driving parameters. The control is provided in such a manner that optimal overall performance of the motor vehicle is achieved with respect to the dynamics of vehicle movement in almost all driving situations. The expenditures with respect to measuring and control techniques for this purpose are low, because the control can largely be integrated into a control apparatus of an antilock brake system (ABS). As a result of the control of speed-dependent tolerance bands for speed differences between the powered axles, an abatement of the control behavior is achieved, as well as soft but sufficiently fast transition behavior between the conditions of an unlimited compensating effect and a completely restricted rotational speed compensation.
Other advantages are obtained when a threshold criterion is adapted for influencing the limiting of the compensating effect of the center differential in the case of wheels with different tread radii. As a result, no additional measuring values or criteria must be used for the disconnecting or reduction of the limiting of the compensating effect. In addition, a cornering adapation system provides that, tight cornering may also be driven without any problems particularly in the lower speed range, and an unacceptably high slip occurring at one axle, is recognized correctly.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.